When I ran The Sugar Hill Invasion for Babies With Knives I needed to make some changes because one of the players wasn't thrilled with the adventure as he'd seen it before.
I've always loved the trope of fallen-cosmic-threat, at least since I saw the Lord of Order incorporated in Kent Nelson's old body in William Moessner-Loaebs' run on Doctor Fate, so I decided that it would be nice to use that as my magic-explaining-device. So I created Emperor Biff, and I offer him to you.
Emperor Biff (aka Empyreanus Bane)
Emperor Biff is an eight-pound long-haired chihuahua. He used to be a being of unimaginable cosmic power and now he is reduced to this. He is under guard/owned-by the significant SHIELD/STAR/PATRIOT/PRIMUS/Argus/whatever organization in your game, the one that deals with cosmic or mystical threats.
A spell has trapped the essence of Empyreanus Bane in this dog. They don't know if the dog is now immortal or not; they don't know what will happen to the essence if the dog dies. Therefore the organization protects him. Keeping him on base with other trapped cosmic beings is less risky than having him in someone's home. (For one thing, the base is armored.)
The dog can talk. He is prone to referring to humans as "pokes of meat and heartbreak" and once referred to his current predicament as "a ton of cosmic mastery in a five-pound sack." He is haughty, mouthy, sarcastic and seems utterly unwilling to escape, possibly because of Rainbow.
Rainbow Timm is the young daughter of Dr. Timm, one of the researchers in the organization. Dr. Timm has no spouse, so Rainbow is often in the complex. Rainbow likes to dress Emperor Biff in doll clothes and have elaborate tea parties. Rainbow and the Empyreanus Bane have formed a connection.
Dr. Timm is not sure whether the connection manages to control Emperor Biff or this is an eventual escape route for Empyreanus Bane. Perhaps when the dog body dies, the Bane will move to the attached victim. Regular scans by a magician show nothing, but...